1. Technical Field
This invention relates to an image display device which displays grayscale images, and in particular relates to an image display device and image display method suitable for display with a large number of grayscales.
2. Related Art
Recent years have seen stunning improvements in the image quality of LCDs (Liquid Crystal Displays), EL (Electroluminescence) displays, CRTs (Cathode Ray Tubes), projection-type display devices, and other electronic display devices.
Devices are being manufactured which have resolution and color gamut substantially comparable to those of human perception characteristics.
However, the reproducible dynamic range for luminance is at most approximately 1 to 102 nits.
Furthermore, 8 bits are used to represent grayscales in general.
In general, the luminance dynamic range which can be perceived by humans at once is approximately 10−2 to 104 nits, and luminance discrimination ability is 0.2 nit.
When a luminance dynamic range is converted into a number of grayscales corresponding to this luminance discrimination ability, it is thought that a quantity of data equivalent to approximately 12 bits is required.
When the display screen of a current electronic display device is viewed via the above-described perception characteristics, the narrowness of the luminance dynamic range is prominent. In addition, grayscale resolution in shadow portions and in highlight portions is insufficient.
For this reason, there is a sense of insufficient realism and vigor in the displayed image.
In CG (Computer Graphics) images used in movies, games, or the like, a tendency has emerged to impart to the data a luminance dynamic range and grayscale characteristic close to those of human perception, in the pursuit of more realistic depictions.
However, because the performance of electronic display devices is inadequate, when displaying the above CG content images, there is the problem in that the expressiveness (the number of bits representing grayscales) inherent in the images of CG content cannot be adequately displayed.
Furthermore, in subsequent versions of Windows (a registered trademark), there are plans to adopt a 16-bit color space, so that the dynamic range and number of grayscales will be greatly increased over the current 8-bit color space.
Hence, there are expected to be mounting demands for electronic display devices with a high dynamic range and large number of grayscales, capable of fully exploiting the 16-bit color space of CG content.
In such electronic display devices, various proposals have been made to expand the above-described luminance dynamic range.
For example, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-99250, in Published Japanese Translation No. 2005-520188 of PCT International Publication, in Japanese Unexamined Patent, First Publication Application No. 2004-317895, and in Japanese Unexamined Patent Application, First Publication No. 2005-258403, configurations are proposed in which a light source in which dimming is possible is used, and as the backlight of a liquid crystal display device, an illumination distribution with illumination differing by area is generated in a form corresponding to the illumination distribution of the video signal (image data), to increase the dynamic range of the video space and increase the number of grayscales, while reducing power consumption.
An important point of each of the above references is the fact that due to the brightness of the backlight, control values (for example, voltage values controlling the transmittance of liquid crystal elements) are set for each pixel of the liquid crystal display device.
Hence, the backlight brightness at each pixel of the liquid crystal display device must be calculated and must be detected.
However, in each of the above references, calculation of the backlight brightness at each pixel is in essence performed using open-loop processing.
In other words, as the backlight brightness at each pixel, predicted values based on numerical values measured in advance are used.
Each of the brightness steps of the backlight, that is, each of the illumination distributions resulting from settings in a control step, is stored in advance in memory as a formula or as a value in a table.
When the backlight is set to a certain brightness, the illumination distribution in areas illuminated by light corresponding to this brightness is calculated using the above-described formula or is read from a table, and the illumination values of corresponding pixel positions are used as illumination values for the pixels.
However, in the methods of the above-described references, when uniform display is performed as in the case of “white” display over the entire image, the illumination distribution of light emitted from the light sources used as individual backlights must be broad to a certain extent, as in a Gaussian distribution, in order to prevent luminance unevenness and false contours, and moreover it is desirable that the illumination distributions of the light sources overlap.
Here, when focusing on a certain pixel, the illumination distributions of all light sources which affect the illumination for the pixel must be considered, to determine the actual illumination for the pixel.
Hence, in the methods of the above-described references, the distribution information or the like for the light sources result in a large amount of data and extreme complexity, and when illumination distributions are to be determined by computation, the circuitry and processing time required for computation are considerable.
Furthermore, when values calculated in advance are stored and are then read out, there is the problem in that large memory capacity is necessary to store illuminations for each pixel corresponding to the above-described distribution information.
In particular, in the methods of the above-described references, processing time for computation is increased, and time is required to read illuminations for each pixel from memory, so that real-time calculation of illuminations for each pixels is difficult, and motion images cannot be displayed.
Moreover, the problem of the need for a large amount of time and memory for the above-described computation processing increases exponentially as the number of light sources and the number of control steps of the light source brightness increase.